Recovery and purification of benzene



Patented Sept. 6, 1960 RECOVERY AND PURIFICATION OF BENZENE Thorwell H.Paulsen, Ashland, Ky., assignor to Ashland Oil & Refining Company,Ashland, Ky., a corporation of Kentucky Filed Apr. 15, 1958, Ser. No.728,620

14 Claims. Cl. 260-674) This invention relates to the recovery andpurification of benzene from crude coke oven or coal tar light oils.

A principal objective of the present invention has been toprovide animproved low cost process for producing high purity sulfur-freenitration grade benzene at high yield from a crude coke oven or coal tarlight oil in which benzene is contained along with toluene, xylene,parafiinic and sulfur-bearing impurities such as thiophenes.

While coke oven light oil is abundantly available at low cost, thesulfur and paraflinic impurities therein have interfered with extensiveuse of this raw material as a source of high grade benzene. caused thechemical industry increased operating costs and lower, yields insubsequent reaction. This impurity problem is so critical thatthechemical industry is switching its high gradebenzene requirements tothe petroleum industry. Also, for this reason the coal tar industry isswitching to the petroleum industrys technique of hydrodesulfurizationto recover benzene from coal tar light oil, using cobalt molybdenumcatalyst, followed by solvent extraction.

At present coal tar producers subject the light oil to an acid treatmentwhich removes some of the impurities, but this treatment does noteffectively remove thiophenes and other impurities and does cause asignificant loss in benzene content. Benzene of suitable purity fornitration purposes or the like cannot be obtained from the acidwashedmaterial by ordinary methods of fractionation, such as might otherwisebe expected, because the boiling point of the thiophenes is too close tothe boiling point of the desired fraction.

In the past various proposals have been made to subject a benzenoidhydrocarbon mixture containing methyl benzenes to catalytic crackingconditions in the presence of hydrogen in the expectation of selectivelyremoving methyl groups from toluene and xylene which may be present,thereby to increase the benzene content of the product being treated.However, the actual experience in this type of procedure falls short ofthe theoretical expectations. As distinguished from dealkylation ofbenzenes having two or more groups of carbon atoms in the alkyl group,the methyl groups from toluene and xylene are reluctant to crack oifselectively. This is particularly true when coal tar light oils form thecharge stock. In the presence of conventional catalysts and at atemperature sufliciently high to effect cracking of impurities, thereaction becomes nonselective and only a small amount of liquid endproduct is obtained along with large quantities of coke and gaseoushydrocarbons. On the other hand, if slightly lower temperatures areemployed, so as to prevent destructive cracking of valuable components,the conversion of methyl benzenes into benzene is very poor, even atprolonged contact time; hence the yield is very low and the necessity ofmulti-pass recycling of the end product prevents commercial utilizationof the procedure for economic reasons.

In recognition of these difficulties, a small volume of benzenepresently is produced from coal tar light oils by a process ofhydrodesulfurization to remove sulfur impurities, followed by solventextraction of the benzene content of the 'desulfurized light oil fromthe end product.

. However, on account of the very high initial cost of in- Theseimpurities have stalling equipment necessary for such treatment, as Wellas the relatively high cost of operating the same, thehydrodesulfurization and selective solvent extraction process affords noeconomic advantage over the production of benzene of equal purity frompetroleum refining sources despite the abundance and low cost of thecrude light oil, and the process is used only because of the high demandfor benzene.

Briefly, the present invention is based upon the discovery that thesulfur impurities such as thiophenes as well as the paraffinicimpurities which are present in a coal tar light oil can be removed, andthe methyl groups can be selectively cracked from the toluene and xylenewhich are also present, without destructive effect on the usefulcomponents of the light oil by subjecting the light oil tohydro-cracking conditions in the presence of a specific catalyst.

More specifically, and notwithstanding past indications to the contrary,the present invention is based upon the discovery that a high yield ofhigh purity benzene can be obtained by subjecting coke oven or coal tarlight oil to catalytic cracking in the presence of hydrogen attemperatures above 1200 F., for example, 1250 F., in a single pass,through a specialized chromia catalyst which is distinguished fromconventional chromia catalysts as subsequently explained.

Otherwise expressed, it has been discovered that the particularcatalyst, hereafter identified in more detail, enables selectivedisproportionation to be accomplished at temperatures considerablyhigher than were heretofore thought possible to employ, and the yield ofthe selective demethylation is so high in a single pass that recyclingof unconverted methyl or alkyl benzene compounds is unnecessary.

More specifically, I have found that a catalyst consisting ofapproximately 10-15% by weight of chromium oxide on a high purity, lowsodium content, gamma type alumina support is capable in the presence ofhydrogen of enabling selective demethylation of benzoid hydrocarbonscontained in coal tar light oil or the like and concurrent conversion ofsulfur-bearing and par-aflin impurities, with little or no cokingeffect, at temperatures above 1200" F. and preferably at approximately1250 F. The catalyst which enables such results to be obtained, incontrast to the poor results obtained in the use of conventional chromiaand other catalysts on convenitional supports, is commercially availablefrom The Girdler Corporation, Louisville, Kentucky under their tradedesignation G-4l. X-ray defraction patterns show the chromia oxide to bepresent in the form of hexagonal crystals as distinguished from chromiaaluminum co-gel catalysts which have also been available but which areincapable of providing similar results. The total chromia content of thecommercial product is calculated as 11.8% Cr O by weight, the remainderof the product being the specified high purity, low sodium contentchromia type alumina. In use the catalyst is employed in the form oftablets, for example, a ma in size, forming a fixed bed through whichthe feed stock and hydrogen are passed continuously.

The process of this invention is effected by contacting the charge ofcoal tar light oil with the catalyst and hydrogen at a temperature'above1200 F., and preferably at a temperature of approximately 1250' F, at acharge flow rate providing a contact time of approximately ten to twelveseconds. Since demethylation of the methyl benzenes present in thecharge stock is accompanied by heat liberation, the feed is preferablyheld at a temperature somewhat lower than the preferred 1250 F.temperature at which the conversion is eifected; for example, wherehydrocracking is to be conducted within the reactor at a temperature of1250 F., the feed temperature may be approximately 1;1S0 F. The feedtemperature provides a convenient means of limiting peak temperature atthe contact zone.

Within the contact zone demethylation occurs rapidly, but even at thehigh temperature indicated there is little destructive cracking ofhydrocarbons into coke or normally gaseous products and little formationof undesirable polybenzenes or biphenyl. In the cracking zone methylgroups split off the toluene and xylene to yield benzene, while thesulfur impurities are converted into hydrogen sulfide and the parafiinimpurities are cracked to gas. The normally liquid products from thehydrocracker are then condensed, and the gaseous products are separatedfrom the liquid products by means of a flash drum and absorber. Dry gasproducts may be used for plant fuel, while the liquid products arestabilized to remove entrained or absorbed gas, and the liquid productsare then subjected to fractionation to separate benzene, toluene, andxylene from one another as desired.

In a typical crude benzol or acid-washed coal tar light oil, the benzenecontent is approximately 60%, the toluene content approximately 20%, thexylene content approximately the balance being paraflins and otherimpurities, the total sulfur content approximating 2000 parts permillion. In terms of yield, the following table illustrates the practiceof the invention on a typical acidwashed coal tar light oil.

MATERIAL BALANCE FOR CRACKING Crude benzene-toluene-xylene Feed:

BTX-- Barrels/ day Benzene 307 Toluene 108 Xylene 44 Total BTX 459Paraflins '41 Total feed 500 Products:

Benzene 336 Toluene 92 Xylene 23 Total BTX 451 Light hydrocarbons:

Ethane and lighter 114.6 million B.t.u./ day.

Propane 2.2 bbls./day.

Butane 21.9 bbls./day.

From this table it will be seen that the benzene yield of the process isin excess of 100%, taking into account the fact that 336 barrels ofbenzene are recovered for every 307 barrels of benzene charged. Thus,substantial disproportionation and little gas or coke loss is obtainedin a single pass through the reactor. At the same time the parafiincontent of the feed is substantially eliminated; the resultant benzeneis essentially sulfur-free and meets commercial specifications fornitration grade benzene.

The temperature at which hydrocracking is effected not only has abearing on the extent to which toluene and xylene are demethylated buthas a bearing on the degree of desulfurization obtained, which, in theproduction of pure benzene, is of great importance. For example, inrespect to a feed stock having a sulfur content of approximately 2000ppm, the following table shows sulfur content of the end product undervarying temperature conditions.

Reactor temperatures: Sulfur content, p.p.m.

A sulfur content of p.p.m. is considered to be essentially sulfur-freein the trade. The benzene produced according to the present invention,whether obtained from coal tar light oil, catalytic reformate, or otherbenzenoid or alkyl benzene sources, is of 99.5+ purity, boils within arange of 0.8" C., and is negativeto the copper strip corrosion test.

One method of practicing the invention continuously on a commercialscale as an adjunct to a petroleum refinery operation to which coal tarlight oil readily can be shipped by rail for conversion is illustratedin the accompanying diagram according to which the hydrogen employed forthe hydrocracking operation is furnished as hydrogen off-gas from acatalytic reformer. This is supplied through line 1 to a gas compressor2. The coal tar light oil is supplied through line 3 to a feed pump 4,and the hydrogen gas and charge stock, commingled in line 5, passthrough a heater 6 wherein the temperature of the mixture is elevated toa temperature sutficient to inaugurate and support selective cracking,or provide desired autocatalytic effect, for example, to a temperatureof 1150 F. or more. This temperature may vary depending upon the weighthourly space velocity in the reactor and may be considerably higher ifthe reactor is provided with a coolant to remove excess heat therefrom.A weight hourly space velocity of 1.0 pound of coal tar light oil perpound of catalyst is suitable but may be increased up to approximately5. The mola-l hydrogen to coal tar light oil ratio may be from 1.0-10; aratio of approximately 5.0 has been found practical. These ratios may bevaried to accommodate variations in the impurity level of the stock.

The hot feed stream passes through line 7 to the reactor 8 wherein apressure of approximately 500 p.s.i.g. is maintained as provided by thecompressor 2 and feed pump 4. The reactor contains a fixed bed ofcatalyst as described through which the reactor feed is passed. Aspreviously indicated, flow rate is preferably adjusted to provide acontact time or time of residence within the reactor of approximatelytwelve seconds. As will be understood by those skilled in the art, thereactor pressure may be varied from approximately 100 to approximately1000 p.s.i.g., the 500 p.s.i.g. pressure disclosed herein being anoperating pressure which is practical to employ.

The reactor output passes through line 9 and a cooler 10 into acondenser 11 wherein the liquid product is condensed. The output streamthen passes through line 12 into a flash drum 13 which may be maintainedfor example at a pressure of approximately 490 p.s.i.g. and atemperature of 100 F. Gases non-condensible under the flash drumconditions of temperature and pressure are liberated from the stream atthis point.

Gases from the flash drum 13 pass through line 14 into an absorber 15which may be held at a pressure of approximately 400 p.s.i.g. andatemperature of 100 F. The absorber operates in conjunctionwith astabilizer 16 and a recirculating liquid product stream'which is passedinto the absorber as lean oil through line 19. Liquid product collectedin the flash drum is led through line 18 to be commingled with fat-oilfrom the absorber in line 17. In this mannervaluable liquid productcontained in the flash drum vapor is recovered in the absorber while thedry gas from the absorber is withdrawn through line 21 for use as plantfuel. The stabilizer may be operated for example at a temperature ofapproximately 155 F. and a pressure of 75 p.s.i.g. The necessary heat ofvaporization for the stabilizer is supplied by reboiler 22 with thevapors returning to the stabilizer through line 23.

A portion of the stabilizer bottom liquid is used as lean oil and ispassed through a cooler. 24 which discharges at a temperature ofapproximately 100 F. to a recirculation pump 25. Also, the stabilizeroverhead, withdrawn through line 26, operates in conjunction with areflux system comprising condenser 27 and receiver 28, the liquid refluxcollecting in the receiver at a temperature of approximately 100 F. andbeing recirculated to the stabilizer by pump 29.

The other portion of the stabilizer bottoms is withdrawn through line 30and passes into a fractionation system comprising one or morefractionating columns which may be of conventional design and whichtherefore are not shown in detail. In the fractionation system shown,benzene is withdrawn from the first fractionating tower 31, the overheadbeing finished benzene, while the bottoms product of fractionater 31 ischarged into fractionater 32 from which toluene is the overhead product.Similarly, the bottoms product from fractionater 32 is passed tofractionating column 33 which provides mixed xylenes as the overheadwhile the bottoms product of fractionater 33 is a heavy end oil whichmay be used as fuel oil. In this procedure the total equipment cost isless than half the amount required for production of benzene of equalpurity by the process of hydrodesulfurization and solvent extractionwhich is now being installed by some coke oven operators and is the onlyknown commercial alternative to such recovery.

It will be noted that the time of contact during which the feed is incontact with the catalyst, for example only ten to twelve seconds asindicated, is very short. It is also apparent, however, that thecatalyst activity is very high inasmuch as the catalyst efiects rapiddemethylation in a single rapid pass of the stock through the fixed bedin this short time period. The unusual and unexpected result is that acatalyst possessing such a high degree of catalyst activity at theprevailing unusually high temperature, is nevertheless selective in itsaction, effectively splitting methyl groups from toluene and xylene andeffectively cracking sulfur-bearing impurities and paraflins withoutdegrating the benzene content of the feed.

While the invention has been disclosed principally in relation to therecovery of purified benzene from coal tar light oil, the invention isalso useful for the treatment of other types of charge stocks containingbenzene such as relatively high boiling catalytic reformate fractions.For example, a catalytic reformate boiling in the 300- 400 F. range,containing 80-85% alkyl substituted benzenes and 15-20% parattins may beused as the charge stock in the practice of the present invention toproduce a hikh yield of benzene, thereby up-grading the commercial valueof the reformate. Thus, the treatment of such feed material at, a rateof 49,000 gallons per day yields 23,400 gallons per day benzene, 11,000pounds per day refined grade naphthalene, and 516 gallons per day of .aheavy residue usable for heavy fuel, as well as 2,837,000,000 Btu. perday of gas.

Having described my invention, I claim:

1. The process of recovering benzene of at least nitration grade purityfrom an impure hydrocarbon stock containing benzene and alkyl benzenecomponents which '6 process comprises subjecting the said stock, in asingle pass, to catalytic cracking conditions in the presence ofhydrogen and a catalyst consisting of approximately to by weight ofchromium oxide on a high purity 5 low sodium content gamma-type aluminasupport, at a temperature above approximately 1200 F. whereby a 7 highyield of benzene is obtained.

2. The process of recovering and purifying benzene from coal tarlightoil, which process comprises subjecting the coal tar light oil, in asingle pass, to catalytic cracking conditions in thepresence of hydrogenand a catalyst consisting of approximately 10 to 15% by weight ofchromium oxide on a high purity low sodium content gamma type aluminasupport, at a temperature above approximately 1200 F. whereby a highyield of the aromatic product is obtained.

3. The process which comprises subjecting a stock containing benzene andalkyl benzene compounds to a catalyst consisting of approximately 10 to15% by weight of chromium oxide on a high purity low sodium contentgamma type alumina support, at a temperature above approximately 1200F., in a single pass, and in the presence of hydrogen for a period oftime not substantially exceeding approximately twelve seconds butsulficient to convert alkyl benzene components into benzene and gas,then separating the gas from the benzene so produced whereby a highyield of benzene is obtained from the said stock.

4. A process of recovering high purity benzene from crude benzolcontaining sulfur and parafiinic compounds which method comprisessubjecting the crude benzol to catalytic hydrocracking conditions, in asingle pass, at a temperature of at least approximately 1150 F. in thepresence of a catalyst comprising 10 to 15% chromia by weight supportedon high purity low sodium content gamma type alumina, for a period oftime suflicient to demethylate alkyl benzene components of the crudebenzol and destructively crack sulfur and paraifinic impurities to gaswhereby a high yield of the aromatic product is obtained.

5. The process of claim 4 wherein the hydrocracking temperature isapproximately 125 0 F.

6. The process of claim 4 wherein the catalytic hydrocracking operationis conducted in the presence of hydrogen ofi-gas obtained from acatalytic reformer.

7. The process of claim 1 wherein the hydrocarbon stock is coal tarlight oil.

8. The process of claim 1 wherein the hydrocarbon stock is a catalyticpetroleum reformate boiling above the gasoline boiling range.

9. The process of claim 1 wherein the hydrocarbon stock is catalyticpetroleum reformate boiling in the range of approximately 300400 F.

10. The process of claim 3 wherein the catalyst contains approximately11.8% chromia on the said support.

11. The process of claim 3 wherein the chromia is i the form ofhexagonal crystals. I

12. The process of claim 4 wherein the hydrocracking operation isconducted by passing a stream of the stock through a fixed bed of thesaid catalyst.

13. The process which comprises selectively splitting methyl groups fromtoluene and xylene and concurrently destructively cracking paraflinscontained in a hydrocarbon stock containing benzene in admixturetherewith which process comprises passing a stream of the stock and astream containing hydrogen gas into contact with one another, in asingle pass, at a temperature above 1200" F. in the presence of acatalyst consisting essentially of approximately 10 to 15% by Weight ofchromia, the

balance of the catalyst composition being high purity low 70 sodiumcontent gamma type alumina whereby a high yield of benzene is obtainedfrom said hydrocarbon.

14. The process of recovering benzene of at least nitration grade purityfrom hydrocarbon stock containing a substantial proportion of an alkylbenzene component 75 which process comprises subjecting the stock, in asingle pass, to catalytic cracking conditions in the presence ofhydrogen and a catalyst consisting of approximately 1015% by Weight ofchromium oxide on a high purity low sodium content gamma type aluminumsupport at a temperature above approximately 1200 F. whereby a highyield of benzene is obtained from said hydrocarbon stock.

References Cited in the file ofthis patent UNITED STATES PATENTS2,705,733 Nonnenmacher et al Apr. 5, 1955 8 2,734,929 Doumani u Feb. 14,1956 2,773,917 Coonradt et a1. Dec. 11, 1956 2,780,661 Hemminger et al.Feb. 5, 1957 5 FOREIGN PATENTS 667,145 Great Britain Feb. 27, 1952 OTHERREFERENCES Alumina Properties, publication by the Aluminum 10 Company ofAmerica 1953 page 5.

1. THE PROCESS OF RECOVERING BENZENE OF AT LEAST NITRATION GRADE PURITYFROM AN IMPURE HYDROCARBON STOCK CONTAINING BENZENE AND ALKYL BENZENECOMPONENTS WHICH PROCESS COMPRISES SUBJECTING THE SAID STOCK, IN ASINGLE PASS, TO CATALYTIC CRACKING CONDITIONS IN THE PRESENCE OFHYDROGEN AND A CATALYST CONSISTING OF APPROXIMATELY 10 TO 15% BY WEIGHTOF CHROMIUM OXIDE ON A HIGH PURITY LOW SODIUM CONTENT GAMMA-TYPE ALUMINASUPPORT, AT A TEMPERATURE BELOW APPROXIMATELY 1200*F. WHEREBY A HIGHYIELD OF BENZENE IS OBTAINED.